Automatic test equipment for checking printed circuit boards has long involved use of a "bed of nails" test fixture in which the circuit board is mounted during testing. This test fixture includes a large number of nail-like spring-loaded test probes arranged to make electrical contact under spring pressure with designated test points on the circuit board under test, also referred to as the unit under test or "UUT." Any particular circuit laid out on a printed circuit board is likely to be different from other circuits, and consequently, the bed of nails arrangement for contacting test points in the board must be customized for that particular circuit board. When the circuit to be tested is designed, a pattern of test points to be used in checking it is selected, and a corresponding array of test probes is configured in the test fixture. This typically involves drilling a pattern of holes in a probe plate to match the customized array of test probes and then mounting the test probes in the drilled holes on the probe plate. The circuit board is then mounted in the fixture superimposed on the array of test probes. During testing, the spring-loaded probes are brought into spring-pressure contact with the test points on the circuit board under test. Electrical test signals are then transferred from the board to the test probes and then to the exterior of the fixture for communication with a high speed electronic test analyzer which detects continuity or lack of continuity between various test points in the circuits on the board.
Various approaches have been used in the past for bringing the test probes and the circuit board under test into pressure contact for testing. One class of these fixtures is a "wired test fixture" in which the test probes are individually wired to separate interface contacts for use in transmitting test signals from the probes to the external electronically controlled test analyzer. These wired test fixtures are often referred to as "vacuum test fixtures" since a vacuum is applied to the interior of the test fixture housing during testing to compress the circuit board into contact with the test probes. Customized wired test fixtures of similar construction also can be made by using mechanical means other than vacuum to apply the spring force necessary for compressing the board into contact with the probes during testing.
The wire-wrapping or other connection of test probes, interface pins and transfer pins for use in a wired test fixture can be time intensive. However, customized wired test fixtures are particularly useful in testing circuit boards with complex arrangements of test points and low-volume production boards where larger and more complex and expensive electronic test analyzers are not practical.
As mentioned previously, the customized wired test fixtures are one class of fixtures for transmitting signals from the fixture to the external circuit tester. A further class of test fixtures is the so called "dedicated" test fixtures, also known as a "grid-type fixture," in which the random pattern of test points on the board are contacted by translator pins which transfer test signals to interface pins arranged in a grid pattern in a receiver. In these grid-type testers, fixturing is generally less complex and simpler than in the customized wired test fixtures; but with a grid system, the grid interfaces and test electronics are substantially more complex and costly. It is the grid-type testers to which the present invention is directed.
A typical dedicated or grid fixture contains test electronics with a huge number of switches connecting test probes in a grid base to corresponding test circuits in the electronic test analyzer. In one embodiment of a grid tester as many as 40,000 switches are used. When testing a bare board on such a tester, a translator fixture supports translator pins that communicate between a grid pattern of test probes in a grid base and an off-grid pattern of test points on the board under test. In one prior art grid fixture so-called "tilt pins" are used as the translator pins. The tilt pins are straight solid pins mounted in corresponding pre-drilled holes in translator plates which are part of the translator fixture. The tilt pins can tilt in various orientations to translate separate test signals from the off-grid random pattern of test points on the board to the grid pattern of test probes in the grid base.
In the past, there has been a need to provide a means of retaining the translator pins in the translator fixture. As mentioned, the fixture typically consists of several parallel translator plates with patterns of drilled holes for retaining a large number of pins extending through the translator plates. The holes drilled in the translator plates are typically drilled at diameters that are slightly oversized with respect to the diameter of the pins, so that the pins may be easily inserted into the holes in the various plates of the translator fixture. The means of retaining the pins in the translator fixture is necessary to prevent the pins from falling out of the fixture if the fixture is lifted up and/or turned upside down without supporting the bottoms of the pins.
There are several prior art approaches to retaining the translator pins in a translator fixture. One approach has been to insert a compressible plastic foam cushion into the space between a pair of plates on the translator fixture. The plastic foam cushion may comprise an open-cell polyurethane foam. The pins are pushed through the plastic foam and, in use, the foam cushion naturally applies a compressible lateral retaining force that holds the pins in place. This approach allows use of straight solid pins which have advantages of tighter spacing capabilities, low manufacturing cost, good probe deflection, and bi-directional use of the pins. However, disadvantages of this approach include the fact that the foam deteriorates over time, drag force from the plastic foam cushion reduces the compliancy of the translator pin force, and the foam cushion is unduly stiffened if translator pin density is high.
Another retention system consists of specially formed translator pins in which each pin has a pair of longitudinally spaced apart enlarged annular rings that project outwardly around the circumference of the pin. The pins are inserted in the translator fixture when the translator plates are assembled. When assembly of the fixture is completed, the rings on the pins are located inboard from the outer translator plates so that the rings can act as stops in preventing the pins from slipping out of the fixture if the fixture is either lifted up or turned upside down. This arrangement has several disadvantages. The projecting rings on each probe reduce the capability of close spacing of the translator pins which can be undesirable if translator pin density must be increased to match a tight density spacing of test points. Further, the translator fixture must be disassembled in order to remove the translator pins each time the pins in the fixture are reconfigured, or for serviceability.
In a more recent approach to the problem, a thin sheet of plastic film such as polyethylene terephthalate (sold under the trademark Mylar) is used as the pin retainer. The Mylar sheet is embedded in the lower translator plate, and undersized holes are drilled in the Mylar sheet in alignment with the larger diameter holes in the translator plates. In addition, undercuts must be formed near the opposite ends of the translator pins, so that when the translator pins are inserted into the fixture, the undercuts are aligned with the undersized holes in the Mylar sheet. The enlarged sections of the pin on opposite sides of the undercut act as stops to prevent the probes from slipping out of the fixture. The undersized holes in the Mylar film act as a retainer and otherwise center the pin in the fixture. The advantage of this approach over use of the foam cushion is that no lateral drag forces are produced by the Mylar film. However, a disadvantage is that it requires specially formed translator pins with undercuts, which greatly increases manufacturing costs when compared with straight solid pins, because of the requirement of grinding the undercuts in each pin.
Other prior art pin retention systems include a fixture having rigid pins with variable diameter sections so that the translator plates can act as stops for the enlarged-diameter sections of the pins. In another fixture an enlarged ring at the bottom of the translator pin fits into openings within a two-part bottom plate that captures a lower portion of the pin within holes in the plates that act as stops on both sides of the ring. Both of these fixtures have the disadvantages that the probes are more expensive, probe deflection is reduced, there is added cost to the fixture, the pins are not bi-directional, pin loading time is greater, and the fixture requires disassembly for reconfiguring the pins or for serviceability. In addition, the fixture requiring the rigid pins with the variable diameter sections adds to the cost of pre-drilling the various holes in the translator plates.
The present invention provides a system for retaining translator pins in a translator fixture that overcomes the disadvantages of the described prior art retention systems. The invention has the advantage that standard straight solid translator pins can be used as opposed to specially formed pins. This greatly reduces the cost of the test fixture as well as ensuring the fixture's ability to match high density patterns of test points. In addition, the invention avoids the problems of drag forces or any other external forces that may otherwise reduce the compliancy force of the translator pins during use. The invention is extremely low in cost and easily adapted to use with all designs of translator fixtures.